As we find more and more planets orbiting other stars, we keep finding ones that are weirder and weirder. Enter GJ 1214b: while much more massive than the Earth, it’s apparently mostly water!

[Click to enhydronate this artists’s illustration.]

The planet — orbiting the star GJ 1214 at 40 light years from Earth — was actually discovered in 2009 by the MEarth project, which is looking for Earth-like planets around, cool, dim red dwarf stars. This is fertile ground for the search: these stars are extremely common, making up something like 80% of the stars in the sky. Not only that, but because they are cool, a planet at the right temperature to have liquid water on the surface would have to be close to the star. That means its period is shorter, making it easier to find (you don’t have to wait a long time for the effects of its orbit on the star to be seen).

In this case, though, it’s not terribly Earthy! First off, it’s massive, weighing in at 2.7 6.5 times our own planet’s mass. It’s also orbiting the star at a distance of a mere 2 million kilometers, giving it a temperature of something like 230° Celsius (450° F): hot enough to roast a chicken.

But it apparently has something the Earth does: water, and lots of it. From our viewpoint, the planet passes directly in front of its star once every 38 hour orbit (this is called a transit). When it does, it blocks the star’s light a little bit (which is how it was discovered). But the planet also has a thick atmosphere, and when it passes in front of the star, that atmosphere absorbs some of the starlight. As it happens, different things in the atmosphere absorb light differently. Water vapor, for example, has a different impact on a spectrum taken of the star’s light than, say, carbon dioxide.

So by breaking the light up into lots of colors and carefully measuring it, it’s possible to figure out what’s in the planet’s atmosphere. Earlier observations could tell that something was absorbing starlight, but they couldn’t tell what. New Hubble observations (PDF) indicate that the best fit to the observations is: water. Haze, for example, absorbs more visible light than infrared, but that’s not what was seen. The spectrum matches the way water absorbs light best, and in fact indicates the atmosphere may be as much as 50% water by mass!

Given the planet’s size of about 35,000 km (22,000 miles), its density is quite low: about 2 grams per cubic centimeter. Compare that to Earth’s density of 5.5 grams per cc! A rocky world more massive than Earth would most likely be denser than 2 grams/cc, so that’s consistent with this planet having lots of water (which has a density of 1 gram/cc).

The scientists involved indicate this planet would be really weird. There may be exotic forms of water there, due to the high temperatures and pressure deep in the planet’s atmosphere. Most likely the planet formed farther out from the star and migrated inward, a phenomenon that is apparently very common in planetary systems. I’ll note other planets like this have been found, too, but not with such a high-precision spectrum and therefore such certainty.

I know that our solar system is pretty weird; we have all manners of strange things floating around in it. But there’s nothing like seeing something so weird it makes us look positively normal by comparison. Sometimes we really do need a swift kick in the planets.

[P.S. My hearty congrats to Zach Berta, the lead author on the Hubble observations. I got to interview him for an Episode 2 of "Bad Universe", and he was very welcoming and fun to hang out with (and got an IMDB credit out of it). We talked quite a bit about GJ 1214b for the interview, and I’m glad to see this planet and its discoverer get some press!]

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This pretty-much puts the nail in the coffin of the “aliens came here to steal our water” storyline. Why mix yourself up with nasty humans when this planet will give you all that you need and no underground resistance movement?

That plus water being the second most common molecule in the entire universe, and the primary constituent of most non-gas giant bodies out past the frost line. The whole “rare water” idea is really one of the odder misconceptions about space in popular culture.

I’m curious, the temperature estimate (I assume) is based on the surface of the planet absorbing a specific amount of solar radiation. If this atmosphere is mostly water vapor, then it could be “clouds”. We know that clouds help hold DOWN the temperature here, by reflecting lots of that solar radiation back out.

Is there any possibility that if this planet is sufficiently cloudy, it could be cooler? (Or, since the atmosphere is mostly water vapor, and water vapor is a greenouse gas, is it more likely to be HOTTER, a la Venus?)

@Christopher Ambler
Who’s to say that GJ 1214b does NOT, in fact, have billions upon billions of nasty humanoids (or “amphibianoids”) and consequently, an “underwater” resistance movement to contend with?

Really wish we could launch probes at a few of these planets and eventually get evidence to confirm the computer models predicting the atmospheric composition. It might take a few hundred years, but it would be awesome for future generations to have confirmation of the model.

7 Earth masses, 2.7 Earth diameters, 7/(2.7)^2 = 0.96. Surface gravity is one Earth gee. Pump some black carbon into the atmosphere and it’s a nice place to live. NASA must immediately, expansively, expensively do studies on parametrically-modeled sequelae to exterminating the GJ1214b biome from the Tepid Teakettle Effect.

Phil, one would be hard pressed to actually know the SURFACE size, hence TRUE density of the planet, as the planet is having a steam bath. That water would largely be steam, which would make the planet puff up by a lot.
Of course, we can easily find out how surface conditions would be like there, just wait a few billion years here, as the sun continues warming, approaching red giant stage.
But, I doubt that the Earth would hold its water as well, as it’s far less massive.
But, we can name the planet: Planet Pressure Cooker.
Or Planet Autoclave…

Purely hypothetical here.
Could an Earth-sized or larger world be composed of nothing but water? Literally, does an Earth-sized drop of water boil away if all other variables remain the same – sunlight, near-circular orbit etc? 10x Earth-sized drop of water?
Up high the low pressure freezes the water, down low it’s liquid, and even lower it starts to boil or do some crazy super-fluid thing right?

As I understand it the debate over the nature of the GJ 1214b atmosphere is still ongoing. There’s a whole load of contradictory evidence, the paper itself notes that the presence of high-altitude clouds or hazes could also explain the data without requiring a water-rich atmosphere.

The question of whether GJ 1214b is a water-world or a mini-Neptune remains open.

@ethanol: The whole “rare water” idea is really one of the odder misconceptions about space in popular culture.
Is it so strange, considering that the greater part of humanity’s worldview is derived from the traditions of desert dwellers?

Actually the paper doesn’t claim to have detected water spectrally – in fact, the opposite:

“…we found the transmission spectrum to be completely flat between 1.1 and 1.7 μm. We saw no evidence for the strong H2O absorption features expected from a range of H2-rich model atmo- spheres.”

What they did find was that the mean molecular weight of the atmosphere is greater than 4: that means that there isn’t an extended H2/He atmosphere, so the low density of the planet can’t be due to those elements. Hence the high water component.

In fact, it isn’t that weird: it’s just a small Neptune, moved close to its star.

@23 Other Paul, really? The greater part of humanity is fairly equally mixed between China and India. Non of the observed religions there are desert dwellers religions.
Seriously though, actually, it was a strange notion, based upon “conventional wisdom” and not by any observations. Just as below 600 feet, life could not exist, because light couldn’t reach that depth. Imagine their shock and amazement when they actually LOOKED down at those depths and deeper. Their amazement and respect for the adaptability of life is readily apparent in the observer’s demeanor.
An interesting thought: If life was in those proposed clouds, would we even recognize it as life?

@Vagueofgodalming, I’ll have to read the paper now. Thanks for “forcing me”.
From the first few seconds of reading: “Rogers & Seager (2010) have proposed three general scenarios consistent with GJ1214b’s large radius, where the planet could (i) have accreted and maintained a nebular H2/He envelope atop an ice and rock core, (ii) consist of a rocky planet with an H2-rich envelope that formed by recent outgassing, or (iii) contain a large fraction of water in its interior sur- rounded by a dense H2-depleted, H2O-rich atmosphere.”
From page 2: “Here, we present a new transmission spectrum of GJ1214b spanning 1.1 to 1.7 μm, using the infrared slit- less spectroscopy mode on the newly installed Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST). Our WFC3 observations directly probe the pre- dicted strong 1.15 and 1.4 μm water absorption features in GJ1214b’s atmosphere (Miller-Ricci & Fortney 2010) and provide a stringent constraint on the H2 content of GJ1214b’s atmosphere that is robust to non-equilibrium methane abundances and hence a definite test of the CH4-depleted hypothesis. The features probed by WFC3 are the same features that define the J and H band win- dows in the telluric spectrum, and cannot be observed from the ground.”
From page 12: “These tests confirm that the presence of the broad H2O feature in the stellar spectrum (see Fig. 2) makes it especially crucial that we employ the detailed, multiwavelength LD treatment.”
From page 13: “A solar composition atmosphere in thermochemical equilibrium is a terrible fit to the WFC3 spectrum; it has a χ2=126.2 (see Fig. 9) and is formally ruled out at 8.2σ confidence. Likewise, the same atmosphere but enhanced 50× in elements heavier than helium, a quali- tative approximation to the metal enhancement in the Solar System ice giants (enhanced 30 − 50× in C/H; Gautier et al. 1995; Encrenaz 2005; Guillot & Gautier 2009), is ruled out at 7.5σ (χ2 = 113.2). Both models assume equilibrium molecular abundances and the ab- sence of high-altitude clouds; if GJ1214b has an H2-rich atmosphere, at least one of these assumptions would have to be broken.”

From page 14: “We find that an atmosphere with a 10% water by number (50% by mass) is disfavored by the WFC3 spectrum at 3.1σ (χ2 = 47.8), as shown in Fig. 9. All fractions of water above 20% (70% by mass) are good fits to the data (χ2 < 25.5). The 10% wa- ter atmosphere would have a minimum mean molecular weight of μ = 3.6, which we take as a lower limit on the atmosphere’s mean molecular weight."
Page 15: "Perhaps most compellingly, a high μ scenario would be consistent with composition proposed by Nettelmann et al. (2011), who found that GJ1214b’s radius could be explained by a bulk composition consisting of an ice-rock core surrounded by a H/He/H2O envelope that has a wa- ter mass fraction of 50-85%. Such a composition would be intermediate between the H/He- and H2O-envelope limiting cases proposed by Rogers & Seager (2010). The H/He/H2O envelope might arise if GJ1214b had origi- nally accreted a substantial mass of hydrogen and helium from the primordial nebula but then was depleted of its lightest molecules through atmospheric escape."

Amphiox, you assume Earth STP upon a world where STP is NOT Earthbound.
As H2O tends to “piss off” things “earthish”, as in rocks and we’ll not even GO INTO things volcanic,n not QUITE.
Volcanoes release many OTHER gasses, dependent upon the nature of the mass in question.
THIS study showed water, based upon extrapolated variables.
The PREDICTION is for the JWST to provide the FINELY REFINED DATA.
Until then, it’s a tantalizing consideration.

Reminds me a lot of the earlier “hot ice” planet Gliese 436 b – click my name for its wiki-page – only smaller and perhaps wetter and rockier.

Not meaning to be a “wet blanket” about this watery world ( ) & I could be mistaken but I think we’ve already found a number of such gas dwarf / Hot mini-Neptunes / “Super-earth” sub-type worlds (that last moniker being a real misnomer here in my view) haven’t we? What makes this one so different or is it just in degree? Not that every new one discovered isn’t a very welcome addition to our knowledge and understanding and interesting in its own right!

Seems we’re starting to get a decent inkling of a whole category of exoplanets that our solar system contains no examples of – just wish we could find out more, more quickly.

If only we had FTL travel & could go visit close-up. Wonder what we’d see and find? Sigh.

Purely hypothetical here. Could an Earth-sized or larger world be composed of nothing but water? Literally, does an Earth-sized drop of water boil away if all other variables remain the same – sunlight, near-circular orbit etc? 10x Earth-sized drop of water? Up high the low pressure freezes the water, down low it’s liquid, and even lower it starts to boil or do some crazy super-fluid thing right?

Yup – I think so – Hot ice forms or so I understand it – different exotic types of ice that remain solid despite extreme temperatures and pressures.

EDIT : Wikipage now linked to my name here – apparently 15 forms of ice with it becoming a metal at the very highest pressures of all akin, I guess, to metallic hydrogen. 8)

Not an expert astrophysicists /astrochemist but I’d speculate that you could have a planet composed of all these cold ‘n’hot ice forms – & later water phases eg. liquid, vapour, clouds* – at various temperature / pressure levels. Finding a purely water planet with no impurities (eg. the odd ingested asteroid or cometary carbon dust & chemical spices) at all strikes me as very unlikely but not entirely out of the question.

Interesting idea if a bit wet!

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* Or would we see clouds if there’s only water vapour and no particulate matter to form the condensation “cores” of raindrops?

Interesting and exciting find! Yet another confirmation that planets are very likely an almost inevitable product of star formation. I still find it frustrating, though, that planets most like our own in size and mass, within the habitable zone of stars similar to our own sun, are by their very nature quite difficult to detect. We still don’t know if such planets are part of the Alpha Centauri system–the closest stars to our own. All we know, so far, is that planets with stable orbits are theoretically possible within the Alpha Centauri system, but that it might take years more of observation to confirm or deny that there are planets there, due in part (according to what I have been able to find on the subject and have been told by some of the regulars to this site–especially MTU), to the dynamical interactions between the stars in that multiple star system.

With a density of 2 gm /cc, this is likely to be a true water world – a world where a rocky interior is surrounded by thousands of miles of ice (not “our” ice, but Ice XI, X, VII), probably a few 100 km of hot liquid (kept from boiling by pressure), and then a steam bath. Look at this phase diagram, and remember that you are starting at 500 K or so, and the pressure increases greatly at depth, so going down into the planet means you are probably following a nearly vertical (but tilted to the right) line on the phase diagram. http://www.lsbu.ac.uk/water/phase.html

it would have to be like the pressure cooker because if the temperature of the water is 500 kelvin, the pressure of the steam at the surface of the world would be higher than 1 atm. Someone out there probably knows the exact water vapor pressure necessary for keeping the water below that liquid at 500 kelvin
so it’s not “waterworld” like I’d think of it, more like steam world

Or would we see clouds if there’s only water vapour and no particulate matter to form the condensation “cores” of raindrops?

There is always particulate matter available. If not in the form of grains of dust, then at least cosmic rays will do. Think of this as an enormous cloud chamber.
Also there is the solar wind. No idea how strong it will be from a red dwarf in 2Gm distance, but for sure it exists.
So yes, there will be condensation if the atmosphere is prepared for rain.

Isn’t 230 celsius within the range of habitability for known extremophiles on earth? Especially in high pressure environments under deep layers of water?

Nope, you are thinking of the <= 130 degC surviving extremophiles, and they can't procreate above 121 deg C (IIRC).

Yes, the pressure keeps the water from boiling. Presumably you can still steam boil the cell content of these critters. Or more likely their membranes can’t take the expansion, a cell wall can withstand a 40 atmosphere differential before it pops. (Which is still a lot. I assume it evolved in an early UCA as a passive means to handle sudden osmotic differentials before cell membrane pumps evolved.)

Instead of comparing temperatures which can have misleading connotations in exoplanetary science (witness the case of Gliese 581c!) and are somewhat difficult to figure out in the absence of more detailed knowledge of the atmosphere, we should really compare irradiation.

From Kundurthy et al. (2011), the luminosity of the star is 0.0028 times solar (this is a bolometric value: red dwarfs radiate most of their energy in the infrared, so the visual luminosity computed from V magnitudes will be lower than this). The orbital distance of the planet is 0.014 AU. Apply the inverse square law: 0.0028 / (0.014^2) = 14 times the irradiation that Earth receives (to 2 significant figures), somewhat greater than the irradiation received by Mercury at perihelion.

I’d also be careful about the assumption that there would be solid “hot ice” in the planet. A naïve three-layer model for the ice giant planets Uranus and Neptune would have them with a hydrogen atmosphere surrounding a “hot ice” shell surrounding a rocky core. However we know from spacecraft observations of their gravitational fields that they have fluid interiors: the mixing between the various components of the planet prevents the ice from solidifying. This also applies to Gliese 436b which was announced as a “hot ice” planet in the media: it appears to have a composition similar to that of our ice giants but more strongly-irradiated, it likely does not have a solid ice mantle either.

Nope, you are thinking of the <= 130 degC surviving extremophiles, and they can't procreate above 121 deg C (IIRC).

Thanks for the reply!

I always thought, though that this 130 C temperature was for 1 atmosphere pressure, while the effective temperature at higher pressures would be significantly higher. But then of course the extremophiles don’t actually live in the very hottest places in the deep ocean vents, but around them, where the temperature gradient falls off.

Given the 14X earth irradiation and the effectiveness of water vapor as a greenhouse gas, the interior of this world could be quite a bit higher than 230C in reality….

Given the 14X earth irradiation and the effectiveness of water vapor as a greenhouse gas, the interior of this world could be quite a bit higher than 230C in reality….

This is what irks me about these temperatures that are quoted regarding exoplanets. If you give someone a temperature value they’ll probably think that it means the number you get if you put a thermometer there.

In fact these temperatures represent the energy balance of the planet: you work out how much energy the planet absorbs from the star, assume the planet is a uniform-temperature blackbody and work out how hot this uniform-temperature blackbody would have to be to balance the incoming radiation. This gives a decent approximation of the temperature for planets with no atmosphere (provided you restrict the region of the planet which is re-emitting the radiation to the daylight hemisphere), but once you have an atmosphere you’re going to get wildly different answers.

For example the equilibrium temperature of Venus calculated this way is 184 K (-89°C), versus the actual temperature at the surface of 737 K (464°C).

The discrepancy for Earth is less but still significant: equilibrium temperature 254 K (-19°C) versus the average temperature of 288 K (15°C). One suggests an iceball, one does not.

(Values above from the NASA planet fact sheets.)

We know that GJ 1214b has an atmosphere, so we shouldn’t expect the equilibrium temperature value to reflect the true conditions there either.

(Incidentally the equilibrium temperature for Venus is lower than that of the Earth: this is because the reflective cloud layer on Venus more than compensates for it being closer to the Sun, the high temperatures there cannot be explained by the increased insolation. The really nasty surface conditions are powered using a lower energy budget per unit area than Earth: with Venus we have an energy-efficient hellworld. Nevertheless I have seen it argued that the carbon dioxide warming in the Earth’s atmosphere is “saturated” and adding more would not warm the planet. Hmmmm.)

Well, whatever sauna fauna the planet has, likely its chemistry will have to be a bit more refractory than what we use. Luckily there are plenty of choices for the basic bits and pieces- perhaps they will be based more on dissolved silica and other minerals. Likely in our own evolution we started out using more of the periodic table, and then CHON became more dominant over time as larger molecules and polymers became available. Even so we still depend on many other elements in more or less trace amounts.

At least when we exchange embassadors there are places here on Earth that they can stay.

Jess @47 – they could stay at Yellowstone, yeah? The lodge would be too cold but they could hang in the hot pools. Or the lake where the hot bubbles are. O, wait, they could stay at the hot pools in Iceland too.

“Given the 14X earth irradiation and the effectiveness of water vapor as a greenhouse gas, the interior of this world could be quite a bit higher than 230C in reality….” [Quoting #45. amphiox -ed.]
This is what irks me about these temperatures that are quoted regarding exoplanets. If you give someone a temperature value they’ll probably think that it means the number you get if you put a thermometer there. In fact these temperatures represent the energy balance of the planet: you work out how much energy the planet absorbs from the star, assume the planet is a uniform-temperature blackbody and work out how hot this uniform-temperature blackbody would have to be to balance the incoming radiation. This gives a decent approximation of the temperature for planets with no atmosphere (provided you restrict the region of the planet which is re-emitting the radiation to the daylight hemisphere), but once you have an atmosphere you’re going to get wildly different answers.

Very good point & comment.

I think we need to be careful of getting ahead of ourselves on things like temperature unless we have direct measures. We can say that the range is likely to be so & so but saying as certainty that temperaures are X when the calculations could well be wrong is a misleading thing that we’re better off not doing.

However we know from spacecraft observations of their gravitational fields that they have fluid interiors: the mixing between the various components of the planet prevents the ice from solidifying. This also applies to Gliese 436b which was announced as a “hot ice” planet in the media: it appears to have a composition similar to that of our ice giants but more strongly-irradiated, it likely does not have a solid ice mantle either.

As you go deep into the interior of Uranus and Neptune you end up with weird superionic phases which essentially behave as plasma. There might be a “solid” core at the centre but the bulk of the planet is fluid. Gliese 436b is predicted to behave in a similar way.

Planets which do not have a significant amount of hydrogen would probably end up forming high-pressure ices though.